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Could a smaller black hole orbit the center of a larger black hole at a distance less than the larger hole's event horizon? What would happen? Seems like nothing unusual but it was an interesting idea.
Almanzo said:Now, one of the stars might be a black hole in its own right.
Chronos said:Concepts of time and space cease to be meaningful inside the event horizon of a black hole.
Interesting proposition.Almanzo said:Consider a swarm of stars, somewhat like a star cluster. If the stars are close enough to their neighbours, and the swarm is large enough, the swarm as a whole will exist inside its own Schwarzschild Radius. (This might not be a stable or long-lasting situation, but it can be conceived of.)
Phrak said:Something doesn't seem right here...
From the perspective of a sufficiently distant observer the event horizons murge. But the question is about a black hole within the event horizon. This requires a different coordinate chart.
xantox said:Nothing forbids the presence of an horizon inside a black hole, however it cannot be an event horizon by definition. But you could find another definition for it, such as a future outer trapping horizon.
Almanzo said:Consider a swarm of stars, somewhat like a star cluster. If the stars are close enough to their neighbours, and the swarm is large enough, the swarm as a whole will exist inside its own Schwarzschild Radius. (This might not be a stable or long-lasting situation, but it can be conceived of.)
MeJennifer said:Interesting proposition.
Any references to the literature?
The actual formal definitions of an event horizon shall be taken from Hawking, or Wald. An event horizon is the future boundary of the causal past of future null infinity, in a weakly asymptotically flat spacetime.Phrak said:I found this (or these) definitions for the event horizon on Wikipedia
A Schwarzschild solution is about empty space, so it cannot apply to the interior of a non-empty black hole such as this swarm of stars.kev said:Assuming that it is possible to concieve of a swarm of stars existing (temporarily) within its own Schwarzschild radius, then the interior Schwarzschild solution would suggest that any black hole at the centre would lose its event horizon.
Exactly.xantox said:A Schwarzschild solution is about empty space, so it cannot apply to the interior of a non-empty black hole such as this swarm of stars.
kev said:Assuming that it is possible to concieve of a swarm of stars existing (temporarily) within its own Schwarzschild radius, then the interior Schwarzschild solution would suggest that any black hole at the centre would lose its event horizon.
xantox said:A Schwarzschild solution is about empty space, so it cannot apply to the interior of a non-empty black hole such as this swarm of stars.
MeJennifer said:Exactly.
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The Schwarzschild and FRW solutions give completely different effects.kev said:A swarm of stars would be loosely described by the interior Schwarzschild solution if you make the aproximation that the mass is distributed evenly rather than concentrated in the stars. The FRW metric for the universe as a whole makes a similar sort of aproximation that the mass of galaxies is evenly spread out in space and ignores the fact that most of the mass is actually highly concentrated in localised regions.
Is that a conclusion that is drawn from GR or just your guess?kev said:A swarm of stars would be loosely described by the interior Schwarzschild solution if you make the aproximation that the mass is distributed evenly rather than concentrated in the stars.
Even if you take a perfect fluid solution, you're still considering an idealized metric which is assumed not to contain any black hole-like object in the first place. So how can you deduce from that anything about the (im-)possibility of black hole-like objects inside this space?kev said:I was careful to specify the interior Schwarzschild solution which covers that part of a spherical non rotating gravitational field that is not empty space.
kev said:Assuming that it is possible to concieve of a swarm of stars existing (temporarily) within its own Schwarzschild radius, then the interior Schwarzschild solution would suggest that any black hole at the centre would lose its event horizon.
xantox said:A Schwarzschild solution is about empty space, so it cannot apply to the interior of a non-empty black hole such as this swarm of stars.
xantox said:Even if you take a perfect fluid solution, you're still considering an idealized metric which is assumed not to contain any black hole-like object in the first place. So how can you deduce from that anything about the (im-)possibility of black hole-like objects inside this space?
xantox said:Nothing forbids the presence of an horizon inside a black hole, however it cannot be an event horizon by definition. But you could find another definition for it, such as a future outer trapping horizon.
kev said:A swarm of stars would be loosely described by the interior Schwarzschild solution if you make the aproximation that the mass is distributed evenly rather than concentrated in the stars. The FRW metric for the universe as a whole makes a similar sort of aproximation that the mass of galaxies is evenly spread out in space and ignores the fact that most of the mass is actually highly concentrated in localised regions.
MeJennifer said:The Schwarzschild and FRW solutions give completely different effects.
kev said:A swarm of stars would be loosely described by the interior Schwarzschild solution if you make the aproximation that the mass is distributed evenly rather than concentrated in the stars.
MeJennifer said:Is that a conclusion that is drawn from GR or just your guess?
xantox said:... (one could better use a FRW solution for such spacetime),,
xantox said:...not because of moving masses, but because the "radius" of such spacetime becomes timelike, so that your argument of checking the time dilation at a given radius seems quite incorrect...
kev said:Assuming that it is possible to concieve of a swarm of stars existing (temporarily) within its own Schwarzschild radius, then the interior Schwarzschild solution would suggest that any black hole at the centre would lose its event horizon.
Phrak said:kev. Perhaps you could back up and explain what you mean by a black hole at the centre.
Are you referring to the singularity of the large black hole, or an additional black hole that fell in?
kev said:I meant an additional independent fully formed black hole that was instantaneously inserted at the centre of the swarm of stars that initially had no mass located exactly at the centre.
Or perhaps better expressed as a small volume of radius r enclosing a mass of r*c^2/(2G) located at the centre of the swarm of stars where the total mass of the stars plus the enclosed black hole is R*c^2/(2G), where R is the radius of the swarm.
An alternative way of looking at it would be to consider a swarm of stars that has a density that marginally less than Schwarzschild density and is about to become a black hole. When the radius of the swarm is 9/8 R_s where R_s is the Schwarzschild radius, an event horizon forms at the centre. By event horizon I mean a region where the coordinate time dilation factor is zero. If you do like the idea of an event horizon that is not located at the Schwarzschild radius of the system then you can think of it as the dynamic zero coordinate time boundary. As the swarm continues to collapse, the zero time boundary (event horizon) moves outwards from the centre until it coincides with the Schwarzschild radius of the swarm, at the exact moment the outermost stars arrive at the Schwarzschild radius. The conventional interpretation is that all the stars then continue to fall to eventually form a region that contains all the mass of the stars within a volume with zero radius at the centre, with infinite density (a singularity).
kev said:It makes me wonder what would happen, if for example we had a distribution of galaxies and clusters within a ten billion light year radius (similar sort of scale to our visible universe) and the total mass of the galaxies was greater than the Schwarzschild density. Would we have no notion of what we normally think of as distance in that sort of a universe?
xantox said:...(so that a black hole interior has absolutely nothing to do with the structure of any common bodies such as stars).
Haelfix said:Worse, there is a problem with the asymptotics. The asymptotics of the interior black hole/star solution does not have minkowski space as a limit, so the very metric itself is poorly joined. In fact, it has some god awful time varying thing as an asymptote.
Chronos said:Concepts of time and space cease to be meaningful inside the event horizon of a black hole.
hurk4 said:Unless someone is inside a BH??
Haelfix said:The problem with having a black hole inside a black hole is several fold.
Xantox is right, there is a problem with the horizon definition.
Worse, there is a problem with the asymptotics. The asymptotics of the interior black hole/star solution does not have minkowski space as a limit, so the very metric itself is poorly joined. In fact, it has some god awful time varying thing as an asymptote.
The problem has indeed been looked at before, and its apparently one of the most excruciatingly complex things to do numerically in all of physics. The last time I talked with someone about it (I believe the state of the art is in Germany), they're still in rarefied extremal D != 4 situations with a bunch of highly technical assumptions which would take a specialist to explain, and even then, the computer returns junk most of the time.
A black hole inside a larger black hole, also known as a nested black hole, is a theoretical concept in which a smaller black hole is contained within the event horizon of a larger black hole.
The formation of a black hole inside a larger black hole is still a topic of debate among scientists. One theory suggests that it could occur when two black holes collide and merge, with one being absorbed into the other's event horizon.
Currently, there is no direct evidence of a black hole inside a larger black hole. However, scientists have observed phenomena that could potentially be explained by the existence of nested black holes, such as gravitational lensing and X-ray emissions.
If a black hole were to enter a larger black hole, it would be stretched and distorted by the intense gravitational forces. Eventually, it would be pulled into the singularity at the center of the larger black hole, adding to its mass and increasing its size.
Once a black hole is inside the event horizon of a larger black hole, it cannot escape. The gravitational pull of the larger black hole is too strong for anything, including light, to escape from within its event horizon.